Wafer level packaging (WLP) is being used extensively for IC packaging. In WLP the die and package are processed and tested on the wafer, prior to singulation. As part of this wafer level process, wafer bumping involves using solder bumps for interconnections and the electromechanical interface for the device. The reflow process is used to form a metallic interconnect phase between the under-bump metal (UBM) and solder. After the solder is deposited on the wafer, the reflow process is used to form homogeneous solder spheres. The prevention of solder oxides during reflow is essential for strength of the interconnection.
The prevention of oxide formation during reflow can be accomplished in numerous ways. Currently the most common approach is to coat the wafer with flux prior to reflow. The coated wafer is then reflowed in a nitrogen atmosphere that further discourages the formation of solder oxides. Time, temperature and atmosphere are critical parameters for proper flux (solder) reflow.
Recently the use of copper pillar microbumps, instead of standard solder bumps, are being used for flip-chip devices to provide further miniaturization in the structure. The process used to form these copper pillar microbumps may leave oxides and hydroxides on the surface of the solder. Gas based processes combined with reflow can be used to clean some metal surfaces, however in many instances a purely gas-based process is unable to remove trace organics left inside the bump. For this reason, spin-coating semiconductor grade fluxes are being used to reform misshapen, uneven, oxide-bearing solder into uniform, coplanar solder microbumps. It is critical for the flux to completely and uniformly coat the top of the solder bump before it enters the reflow process.
The reflow temperature profile is critical for the process to succeed. The wafer must be exposed to the maximum temperature for only the shortest effective period during its time in each temperature zone. The overall profile contains four phases: preheat, pre-reflow, reflow and cooling.
Lead-free solders require higher process temperatures. Reduction in solder paste volumes require narrower process windows to optimize the reflow profile.
Low oxygen levels (<50 ppm) are critical to successful reflow since the presence of oxygen causes oxidation to the joins resulting in bad solder joint reliability.
Currently the solder reflow process is done in separate pieces of equipment to coat, reflow and clean. This equipment takes up a large amount of expensive floor space in the semiconductor fab. FIG. 1 shows a conventional system that is made up of multiple pieces of equipment.